1. Field of the Invention
The present invention relates to a rotor of an electric motor which has magnets, an electric motor, and a method of production of a rotor of an electric motor.
2. Description of the Related Art
In general, an electric motor of a type which attaches permanent magnets to the surface of a rotor core is called an SPM (surface permanent magnet) motor. The permanent magnets of an SPM motor are acted on by the centrifugal force due to the rotational motion of the rotor and the attraction force due to the magnetic force of the stator outward in the radial direction. For this reason, to prevent the permanent magnets from flying off due to operation of the SPM motor, a fastening method which fastens the permanent magnets to withstand the above centrifugal force and attraction force has been sought. In many cases, the permanent magnets of the SPM motor are bonded by a binder, for example, an epoxy resin, to the rotor core. However, the bond strength by the binder is not necessarily sufficient, so it is necessary to limit the output torque of the motor so that the above centrifugal force and attraction force are kept down.
As another fastening method of permanent magnets, Japanese Patent No. 5082534 proposes a method comprising forming a plurality of holding projections with T-shaped cross-sections vertical to the axis of rotation of the rotor in the circumferential direction of the rotor core and arranging permanent magnets between the adjoining holding projections so as to lock the two end parts of the permanent magnets. However, in the method of Japanese Patent No. 5082534, if reducing the curvature rates of the permanent magnets so as to realize a larger torque, since the holding projections project outward in the radial direction from the surfaces of the permanent magnets, there is the danger of the holding projections contacting the stator. On the other hand, if increasing the distance between the stator and rotor so as to prevent contact of the two, the result is that the magnetic gap becomes larger, so the output torque of the motor can fall. Further, the holding projections of Japanese Patent No. 508253 are magnetic bodies which are formed integrally with the rotor core and can absorb the magnetic flux of the permanent magnets, so the magnetic flux near the holding projections can short-circuit regardless of the stator. As a result, the magnetic flux density of the motor decreases and the output torque falls.
A rotor of an electric motor which reliably fastens the magnets to the rotor core and which prevents a drop in the output torque has been sought.